The invention concerns heating of hydrocarbon materials in geological subsurface formations by radio frequency electromagnetic waves (RF), and more particularly to heating by RF energy emitted from one or more polygonal antennas.
Extraction from heavy oil reservoirs including oil sands deposits, shale deposits and carbonate deposits, requires heating of the deposits to separate hydrocarbons from other geologic materials and to maintain hydrocarbons at temperatures at which they will flow. Known methods of heating such deposits include steam heating, electric resistance heating and heating by RF energy.
Heating subsurface heavy oil bearing formations by prior RF systems has been inefficient due to traditional methods of matching the impedances of the power source (transmitter) and the heterogeneous material being heated, uneven heating resulting in unacceptable thermal gradients in heated material, inefficient spacing of electrodes/antennae, poor electrical coupling to the heated material, limited penetration of material to be heated by energy emitted by prior antennae and frequency of emissions due to antenna forms and frequencies used. Antennas used for prior RF heating of heavy oil in subsurface formations have typically been dipole antennas. U.S. Pat. Nos. 4,140,179 and 4,508,168 disclose prior dipole antennas positioned within subsurface heavy oil deposits to heat those deposits.
Arrays of dipole antennas have been used to heat subsurface formations. U.S. Pat. No. 4,196,329 discloses an array of dipole antennas that are driven out of phase to heat a subsurface formation.